The repair of tendons or ligaments is a challenging and complication prone area of surgery. As one example, the dilemma in flexor tendon repair surgery in the hand is to adequately connect a severed tendon without compromising the functionality of the hand due to surgical intervention and repair techniques. Over the past 40 years, there have been only improvements in the basic suture techniques to repair tendons. In order to make any substantial improvement in the art of repairing a severed tendon one must first understand the composition and structure of tendons and ligaments.
Tendons can sustain high tensile forces resulting from muscle contraction, yet are flexible enough to bend around bony surfaces and deflect beneath retinacula to change the final direction of muscle pull. Tendons attach muscle to bone and transmit tensile loads from muscle to bone thereby producing joint movement. Ligaments attach bone to bone and can flex to allow natural movement of the bones that they attach, but are strong and inextensible so as to offer suitable resistance to applied forces. Ligaments augment the mechanical stability of the joints. The biomechanical behavior of tendons and ligaments is viscoelastic or rate dependent, that is, their strength and stiffness increase with an increased loading rate. Bundles of collagen fibers embedded in a connecting matrix, known as ground substance, provide the load carrying elements of natural tendons and ligaments. The arrangement of the collagen fibers is nearly parallel in tendons, equipping them to withstand high unidirectional loads.
The less parallel arrangement of the collagen fibers in ligaments allows these structures to sustain predominant tensile stresses in one direction and smaller stresses in other directions. The ground substance in both tendons and ligaments acts generally as a cementing matrix holding the collagen fibers together. The ground substance retains large amounts of water essential to the non-compressive hydraulic function of the moving tissue. Also included in the tendon composition are elastic fibers, tenocytes, small blood vessels and nerves. In general, the cellular material (fibroblasts) occupies about 20% to 38% depending on references, of the total tissue volume, while the ground substance matrix accounts for the remaining 62% to 80%. About 70% of the ground substance matrix consists of water absorbed in an open polysaccharide matrix.
Two types of tendons exist in the hand for connecting phalanx (finger) bones to the appropriate muscles. Flexor tendons, which are connected to the volar or palm side of the fingers, lend the ability to curl the fingers towards the palm. Extensor tendons, which are connected to the dorsal or backside of the fingers, return the curled fingers back into a straight position. Sheaths and retinacula restrain most tendons in the hand to some extent and keep them close to the skeletal plane so that they maintain a relatively constant moment arm rather than bowstringing across the joints. The pulley system of the flexor tendon sheath in the finger is the most highly developed of these restraints. The flexor tendon sheath pulley system permits the flexor tendons to maintain a relatively constant moment arm and helps minimize stress risers between tendon and sheath. This system serves three important functions. First, it allows smooth tendon gliding or lubrication; second, the retinacular reinforcing pulleys maintain the flexor tendons close to the surface of the finger bones, preventing bowstringing; and third, it provides an enclosed synovial fluid environment for tendon nutrition and lubrication. As the finger moves, each tendon slides a certain distance, which defines the “excursion of the tendon”. Excursion takes place simultaneously in the flexor and extensor tendons during joint motion. The tendons of the agonist, or contracting muscle, displace in one direction. The tendons of the antagonist or resisting muscles displace in the opposite direction to accommodate the motion.
Today, the most common methods of repairing torn, severed or otherwise damaged tendons involve approximating the severed ends of the tendons and suturing one side of the tendon to the other thereby returning the tendon to its natural position. A popular suture technique is the so-called Kessler technique and slight modifications thereof. Some of the other techniques include the Becker, Savage, lateral trap, double loop locking suture, four-strand interlock and variations of the Halsted technique. Other methods place prosthetic material either within or around the tendon. Polyester strips and sleeves along with polyester mesh have been used to reinforce the suture/tendon interface to provide a stronger repair.
After flexor tendon repair, resistance to tendon gliding increases at the repair site. Repair techniques that use an increased number of suture strands, or increased amounts of suture material or prosthetic material promote greater glide resistance. In particular, adhesions form due to the tendon's natural response to healing, i.e., the ingrowth of cells and vessels from surrounding connective tissue. Current literature suggests adhesions may constitute an inflammatory process at the site of repair and an extension of the intrinsic tendon healing process to the surrounding tissue.
An ideal repair would exhibit high strength, flexibility, and a joining of the tendon ends without any foreign material on the outside surface of the tendon. Physical therapy should begin immediately after the repair to prevent the tendon from adhering to the tendon sheath creating adhesions that limit the full excursion of the tendon in its sheath. For this reason, the repair site must withstand the immediate tensile stress being applied to it during physical therapy. In a relaxed state, a flexor tendon experiences about one pound of constant tension. When a person applies a light grip, such as by grasping a key, about three to four pounds of tensile force is applied to the tendon. A strong grip can apply over ten pounds of tensile force to a tendon.
Since most suture-based tendon repairs reach their tensile limit at about 6 lbs., surgeons must balance the desire to have full and immediate active motion to prevent adhesions against the need for immobilization to prevent rupture of the repair. Earlier loading of a repaired tendon promotes a more rapid increase in repair strength. For a tendon to properly rejoin, the opposed tendon ends do not have to touch but they do need to be approximated within 1-2 mm of each other to properly reattach. An ideal tendon repair would hold the lacerated tendons together to begin healing and tissue generation but slowly release tension allowing the tendon to become the primary load bearing structure. Tendons will heal at a rate that is proportional to the load being applied during physical therapy.
Another major problem is the softening of the damaged tendon ends, which begins shortly after the damage or injury occurs and continues for approximately the next twelve days. This softening results in a weakening of the tendon fibers, which contributes to the formation of a gap at the repair site during the early phases of tendon healing. It is believed that gaps form at the site of repair due to a loss of purchase by the grasping portion of the suture at the tendon-suture interface. The grasping suture may even completely tear out, resulting in a failure of repair. A term for this failure is “rake-out”. Rake-out is a failure mode associated with suture tendon repair in which the end of the severed tendon has weakened and the suture tends to pull out of the tendon ends. This splits the tendon and results in an undesirable gap or total failure. Another common type of suture repair failure is of a suture knot.
The effectiveness of a suture depends on many factors, such as the suture material, the technique with which the suture is inserted, and knot strength. Immediately after a tendon is repaired, the strength of the repair depends almost entirely on the suture technique. The ideal suture knot should terminate securely, be strong, easy to handle and inelastic. The suture material used today is generally braided polyester or a monofilament polypropylene. Using current suture techniques, absorbable suture materials do not have enough residual tensile strength over time to resist gapping and rupturing. The ideal suture technique should be easy to use, minimize interference with tendon vascularity and be completely internal to the tendon without increasing the bulk of the tendon. Locating the knots outside the tendon rather than within the repair site may result in higher ultimate tensile strength but will also increase the risk of adhesions and increase the friction through the pulleys. This latter characteristic is known as “work of flexion”.
Most suture methods employ an internal suture with external knots distal and proximal to the laceration or within the laceration. The surgeon typically uses a continuous running external suture at the junction of the repair, known as an epitendinous suture, to approximate the tendon ends. The use of the epitendinous suture increases the tensile strength of the repair and helps to resist gapping, but it can also increase the risk of adhesions and is difficult to master and very tedious to execute. The evolution of tendon repair with sutures starts with the two-strand technique. Some of the variations of this technique are the Bunnell, Kessler, and Tsuge methods. When two-strand repairs fail, the failure usually occurs at the knots. Studies have shown that the initial strength of these repairs is proportional to the number of suture strands that cross the repair site. This has led to a trend of doubling, tripling, and even quadrupling the number of strands placed across the repair site. With these multiple strand techniques, Savage, Becker and Ketchum have shown significant tensile strength over the two-strand methods but they are more difficult to perform and add material to the outside surface of the tendon with more exposed knots. These techniques focus primarily on the increased effect on tensile strength and disregard the increased resistance to the tendon gliding through the pulleys. Therefore, the quest continues for the ideal suture technique having the tensile strength required to allow the patient to start physical therapy immediately, and having the low profile necessary to minimize adhesions that compromise the ability of the tendon to glide through the pulleys.
Techniques have also been developed that incorporate an internal or external prosthetic splint. Low porosity woven polyester, which is the same material used for aortic graft repair, is being used as an artificial splint. There are basically two methods of splint repair. The internal splint technique is accomplished by placing a horizontal slit transversely in each tendon stump proximal and distal to the laceration site. A rectangular piece of polyester splint is placed into this slit on both sides of the tendon. Sutures are then placed perpendicular to the graft along each tendon thereby attaching the splint to the tendon. The sutures attach the splint, which is basically a flexible tensile member, to the interior surface of the tendon. These suture knots are then tied on the outside of the tendon for ease of placement and an epitendinous suture is placed at the junction of the repair. As previously mentioned, the external knots will increase the risk for adhesions and also increase the work of flexion. The material of the tendon splint is inert and similar to the suture material being used in other techniques and its internal position within the substance of the tendon should promote tissue ingrowth and enhance the repair site. However, the large slits in the tendon ends might structurally damage the internal blood supply of the tendon and cause tissue degeneration.
In the external splint technique, also known as the dorsal tendon splint technique, the surgeon aligns both tendon ends and places a two-strand Savage type core suture on the anterior surface of the tendon. The surgeon then places a rectangular Dacron® splint on the dorsal surface of the tendon across the laceration site and sutures it to both tendon ends. In this technique, and as mentioned earlier, the splint acts as a flexible tensile member that prevents the tendons from gapping and rupturing during early movement. As with the internal method, the knots are placed on the exterior surface of the tendons and the splint is actually on the outside surface. This will increase the risk of adhesions and consequently increase the work of flexion. The internal tendon splint may add too much bulk to the repair site, and the external tendon splint may interfere with tendon gliding. Preliminary work of flexion studies suggest both tendon splints increase the work of flexion by 16-19%.
Another splint-type technique being used today is a Dacron® or Prolene® mesh sleeve that surrounds the tendons. The two ends of the lacerated tendons are placed in the proximal and distal openings of the sleeve. The tendon ends are butted together without any additional sutures, except that an epitendinous suture is placed thereby attaching the sleeve to the outside surfaces of the tendons. This is done on both ends of the sleeve. This technique is 117% stronger in tension than a conventional two-strand core stitch technique with an epitendinous suture on the external surface. Like the aforementioned splint techniques, these are tested in vitro (outside of the body) and do not take into account any of the in vivo (inside the body) problems that occur such as placing a significant amount of repair material external to the tendon and within the tendon sheath. Again, external repair material provides a potential source of fibrous adhesions and an increase in work of flexion.
Implanted anchors have also been used to attach two ends of a severed tendon. This type of anchor is similar to a Dacron® splint in concept but is usually fabricated from stainless steel or titanium. The geometry of the anchor also differentiates the anchor from a splint. The anchor, which may measure 20 mm in length, 3 mm in width and 1 mm in thickness, has a symmetrical double barbed end configuration. The anchor is placed into the severed end of the tendon by making a small transverse incision. Once the anchor is in the correct depth the surgeon will place a suture through the tendon at the flat side of the barb and knot the suture into a loop thereby preventing the barb from being pulled out of the tendon. The tendon will be sutured at each flat on the barb, providing two suture loops per tendon end. The same suture technique is performed on both ends thereby re-attaching the severed tendon. This repair technique shows an increase in mean ultimate tensile strength of 49-240% over traditional two-strand and multi-strand suture techniques. This technique is relatively easy to perform but it does not address the in vivo problems caused by placing the suture knots on the outside of the tendon. Here, they become a potential source of fibrous adhesions and increase the work of flexion. This type of tendon anchor can limit motion or cause pain when positioned directly over a joint with the finger in maximal flexion since it is long and fairly rigid. Also, the surgeon must still bring the tendon ends together with a separate surgical tool and, in the process, risk damaging the tendon ends.
Adhesives have been evaluated in the search for the ideal tendon repair. Studies have been conducted using adhesives of the cyanoacrylate group, more commonly known as super glues. These adhesives form a strong adhesive bond with most human tissue, particularly those containing a large amount of protein, such as skin and tendon tissue, because they polymerize in the presence of water and hydroxyl groups, both of which are abundantly present in tendon tissue, and they do not require a solvent. They are known to be biodegradable, although the time taken to degrade in tendons is unknown and only the long chain varieties are known to be minimally toxic to human tissue. The application of adhesives in tendon repair is in conjunction with two-strand or multi-strand core suture with an epitendinous suture. The adhesive is placed on the tendon ends after the sutures have been placed and approximated to allow for polymerization. The shortcomings that were discussed in connection with suture repair are experienced with adhesive techniques as well. Some problems with adhesives include their potential non-biodegradability within the tendon, their questionable effect on tendon healing, and their potential local and systemic toxicity. Currently, therefore, adhesives do not provide an adequate solution to tendon repair problems.
Current and past tendon or ligament repair techniques concentrate on increasing the tensile strength of the repair by adding more structural components to the repair, e.g., sleeves, splints, additional suture strands, additional knots and adhesive. All of these techniques trade off between early tensile strength, increased work of flexion, and increased risk of adhesions or other problems. While the surgeon debates the clinical technique, the patient may suffer from a less than desirable outcome and discomfort over the life of the repair. Adhesions cause pain and limit motion of the affected joints. By increasing bulk to the tendon, motion may be further limited and this can result in a defect called “trigger finger.”
None of these techniques have utilized the physiological makeup of the tendon to provide a stronger repair. The tensile strength of the tendon is provided by the lengthwise parallel collagen fibers, which give it the ability to withstand high tensile loads. The ground substance is made up primarily of water and cannot be used to provide strength to the repair. The tendon sheath is also too weak to provide meaningful assistance with holding the two tendon ends together.
Similar problems arise when attaching tendons or ligaments to bone. That is, simply suturing the tendon or ligament to a bone anchor or using external tendon anchor members may not provide the necessary strength of repair. As further discussed above, these techniques also promote adhesion formation.
Finally, tendon retrieval has also been a problematic portion of tendon repair surgery. Typically, the surgeon must use a small grasping tool with thin, movable jaws similar to needle-nose pliers to grasp a tendon end and pull and transfix it in an appropriate operating position. Unfortunately, gripping the tendon ends in this manner often damages them and makes the tissue less able to hold the epitendinous suture. The damaged tendon ends will also form scar tissue or adhesions which further adversely affect the repair.
Therefore, there is a need for tendon repair techniques and apparatus that harness the intrinsic strength of the tendon fibers, but allow the tendon to flex while moving through the sheath. This repair apparatus should resist any gapping or rupture during immediate post-operative physical therapy, and reside in the interior of the tendon to reduce or possibly eliminate post-operative adhesions. The repair apparatus should also produce low work of flexion while gliding unhindered through the tendon sheaths. There is generally a need for tendon repair apparatus and methods that allow the patient to immediately begin active physical therapy without risking any tendon repair failure while minimizing or eliminating the need for sutures or other repair structure on the external surfaces of the tendon thereby reducing the occurrence of adhesions and friction between the tendon repair and the sheath pulley. There is a further need for tendon-to-bone repair techniques and apparatus with at least some of these attributes. Finally, there is a need for a tendon retrieval device which also harnesses the inherent strength of the tendon fibers and minimizes damage to the retrieved tendon end.